The present invention relates to methods for reducing fluorescence in paper-containing samples.
An important aspect in the manufacture of paper products is the whiteness of the paper. Traditionally, one means of increasing the whiteness of paper has been through the use of fluorescent whitening agents (FWAs). FWAs absorb light in the near ultraviolet (UV) and emit this light in the visible spectrum, thereby increasing the amount of visible light reflected from the paper, and thus increasing the apparent whiteness of the paper.
The presence of FWAs in manufactured paper, however, is limited by federal regulations, particularly in paper products destined to contact food products. Federal Regulation 21 CFR xc2xa7176.260 prohibits the presence of any harmful or deleterious materials that can migrate to food products from recycled papers used for food packaging.
Recycled paper is frequently used in the manufacture of new paper products, which poses several potential problems. First, recycled paper often contains FWAs, which, as discussed above, are forbidden in products destined for food-contact. Second, recycled paper arises from many different sources and the whiteness of the final manufactured product will vary dramatically depending on the amount of FWAs present in the starting materials. Third, for optimal performance of a paper mill, processing recycled paper requires the amount of fluorescence in the pulp going to the mill to be constant. Therefore, the recycling of paper products which have been previously treated with FWAs cannot be undertaken without an effective method for destroying the FWAs (Dubreuil, Progress in Paper Recycling, 1995, August, 98-108).
The principal FWAs used in the paper industry are diaminostilbenes which have the general structure represented in Formula I: 
One common FWA is Tinopal SFP, which has the structure of Formula I wherein R1 is NH2 and R2 is NH(CH2CH2OH). Other potential R1 substituents include, but are not limited to, N(CH3)(CH2CH2SO3H), and OCH3. Other potential R2 substituents include, but are not limited to, N(CH2CH2OH)2, NH(C6H4SO3H), and N(CH3)(CH2CH2SO3H). Other FWAs are known and have the general structure as shown in Formula II, below. 
Several techniques are known for the removal of FWAs from recycled paper. For example, oxidizing agents, such as chlorine, destroy the stilbene dyes which make up most FWAs, rendering them nonfluorescent. The destruction of FWAs can be assessed by monitoring the fluorescence of a sample. As the FWAs are destroyed, the sample""s fluorescence decreases. Increasingly stringent environmental regulation of organic halides imposes limits on such use of chlorine in the removal of FWAs from paper-containing samples.
Other non-chlorine reagents have been evaluated for their potential use in the destruction of FWAs, including sodium hydrosulfite, formadine sulfinic acid, and hydrogen peroxide; however, each of these had the reverse effect of actually increasing the measured fluorescence of the samples. Ozone has also been used as an agent to destroy FWAs. Although ozone does reduce the fluorescence of test samples, unacceptably high levels of ozone are required to reduce fluorescence. Ozone also has the undesirable effect of degrading cellulose, resulting in a paper product with decreased strength properties. These disadvantages, coupled with the high cost of ozone, make the use of ozone impracticable.
There is, therefore, a need for methods of reducing fluorescence in paper-containing samples that do not suffer from the drawbacks of existing methods. There is a further need for methods of reducing fluorescence in paper-containing samples that contain FWAs. There is also a need for improved methods of destroying FWAs in paper-containing samples.
The present invention is directed to novel methods for reducing fluorescence in paper-containing samples. According to the methods of the present invention, a paper-containing sample is combined with an oxidoreductase, a mediator, and oxygen to form a mixture, which mixture is then optionally incubated, resulting in reduction of fluorescence of the paper-containing sample.
The methods of the present invention may also be used for destroying FWAs in a paper-containing sample.
The present invention arises from the surprising discovery that the amount of fluorescence in a paper-containing sample may be reduced by mixing the paper-containing sample with an oxidoreductase, a mediator, oxygen, and optionally heat. The present inventors have discovered that FWAs present in a paper-containing sample may be destroyed by using this method.
As used herein, the term xe2x80x9cfluorescence,xe2x80x9d also referred to as the fluorescent component of a sample, is expressed in terms of the number of brightness units (0ISO) emitted by a sample. Fluorescence is the radiation emitted as the result of the absorption of incident radiant energy of different wavelengths. When irradiated by UV, the fluorescent radiation is often visible. The higher the fluorescent component, the more fluorescence is being emitted by the sample.
As used herein, the term xe2x80x9creduce fluorescence,xe2x80x9d and variations thereof, are used herein synonymously with inhibit, reduce, suppress, decrease, diminish, and lower fluorescence. The present invention includes methods that substantially inhibit fluorescence. Substantial inhibition of fluorescence refers to a reduction of fluorescence from about 1% to about 100% compared to the untreated paper-containing sample comprising FWAs. Preferably, fluorescence is inhibited up to about 50%, more preferably up to about 75%, and even more preferably up to about 100% (i.e. such that the fluorescence of the treated paper-containing sample is indistinguishable from background).
As used herein, the term xe2x80x9cbackgroundxe2x80x9d refers to the level of fluorescence emitted by a paper-containing sample containing no FWAs. A typical paper-containing sample comprising FWAs may have a fluorescent component of about 5. Following treatment of such a paper-containing sample by the method of this invention, the fluorescence of the paper-containing sample may drop to about 1. Thus, treatment has resulted in a reduction of fluorescence in the paper-containing sample.
The fluorescent component of a paper-containing sample is obtained by measuring the brightness for a specified blue light measured under standardized conditions, with and without UV illumination, on an instrument designed and calibrated for this purpose. Brightness without UV illumination is subtracted from brightness with UV illumination, yielding the fluorescent component. 0ISO brightness is one of the standardized reflectivity techniques used for pulp, paper, or paperboard.
As used herein, the term xe2x80x9cpaper-containing samplexe2x80x9d refers to a sample which contains paper. As used herein, the term xe2x80x9cpaperxe2x80x9d refers to all kinds of sheets of fiber formed on a support screen from a water suspension and includes sheets formed by other methods. Paper may be recycled or non-recycled, and may comprise pulp from both natural and artificial sources.
As used herein, the term xe2x80x9crecycledxe2x80x9d refers to paper and/or paper by-products which have previously been used. xe2x80x9cRecycledxe2x80x9d may refer to samples containing only previously used paper and/or paper by-products as well as to samples containing previously used paper and/or paper by-products mixed with non-recycled paper. In a preferred embodiment, the xe2x80x9cpaper-containing samplexe2x80x9d comprises recycled paper and FWAs.
As used herein, the term xe2x80x9cpulpxe2x80x9d refers to mechanically ground or chemically digested materials, especially wood. Paper pulps are composed of fibers and are often prepared from wood, cotton, or grasses using chemical or mechanical processes. Synthetic pulps are also known to those of skill in the art.
As used herein, the term xe2x80x9cfluorescent whitening agentxe2x80x9d (FWA) refers to any material which increases the brightness of a sample. xe2x80x9cFluorescent whitening agentsxe2x80x9d include optical brighteners (OBs), optical brightening agents (OBAs), and fluorescent brighteners (FBs). Examples of FWAs include, but are not limited to, Tinopal SFP.
In accordance with the method of the present invention, an oxidoreductase is added to paper-containing samples to reduce fluorescence and/or destroy FWAs. As used herein, oxidoreductases include, but are not limited to, laccase, manganese peroxidase (MnP), lignin peroxidase (LiP), and chloroperoxidase (CP). In an embodiment of the present invention, an oxidoreductase is added to the paper-containing sample. In another embodiment, two or more different oxidoreductases are added in combination or sequentially to the paper-containing sample according to the present method to reduce fluorescence.
Oxidoreductases may be added to the paper-containing sample at concentrations ranging from about 14,000 units (U) per gram of oven-dried pulp (O.D. pulp) to about 1.5xc3x97106 U per gram of O.D. pulp. In a preferred embodiment, from about 20,000 U to about 560,000 U of oxidoreductase is added per gram of O.D. pulp. In a more preferred embodiment, about 24,000 U of oxidoreductase is added per gram of O.D. pulp. In another preferred embodiment, the oxidoreductase is laccase.
As used herein, the term xe2x80x9coven-dried pulpxe2x80x9d is well known to those skilled in the art and refers to the solid component of pulp that results from oven-drying a pulp sample. For example, if 4 grams of pulp are oven-dried, the weight after drying might equal 1.44 grams or 36.0% of the original pulp sample""s weight. If a protocol required an amount of pulp equivalent to 20 grams of oven-dried pulp, one would add 55.6 grams of pulp sample (20 grams divided by 0.36).
A mediator is also added to the paper-containing sample to reduce fluorescence. As used herein, mediators include, but are not limited to, Nxe2x80x94OH containing compounds. Mediators are small organic molecules which, along with optional enzyme and oxygen, are believed to be needed to oxidize the substrate. It is believed that the oxidoreductases oxidize the mediator, which becomes the active species in the system. The active species can then disrupt the fluorescence caused by the FWAs. Nxe2x80x94OH compounds include, but are not limited to, 1-OH-Benzotriazole (HBT), Nxe2x80x94OH-Acetanilide (NHAA), Violuric Acid (ViO; 5-(hydroxyimino) barbituric acid), N-hydroxyphtalimide, and 2-2xe2x80x2 azinobis (3-ethylbenzthiazoline-6-sulfonate (ABTS). The chemical structures for several mediators are shown in formulas III-VII below. 
In an embodiment of the present invention, one mediator is added to the paper-containing sample. In another embodiment, mixtures of two or more different mediators are added to the paper-containing sample. In a preferred embodiment, HBT is added to the paper-containing sample at a percent charge from about 0.5% to about 10%. As used herein, the term xe2x80x9cpercent chargexe2x80x9d refers to the amount of mediator added to the system per gram of O.D. pulp. In a more preferred embodiment, HBT is added to the paper-containing sample at a percent charge from about 1% to about 4%. In an even more preferred embodiment, HBT is added to the paper-containing sample at about a 2% charge.
In accordance with the present invention, oxygen is added to the paper-containing sample. It is believed that oxygen triggers the oxidation of the mediator by the oxidoreductase. Oxygen may be added as a gas. In an embodiment of the present invention, oxygen is added to the paper-containing sample at about 1 to about 20 bars. In a preferred embodiment, oxygen is added to the paper-containing sample at about 5 to about 15 bars. In an even more preferred embodiment, oxygen is added to the paper-containing sample at about 10 bars. Oxygen may be added to the mixture in a pressurized vessel.
As used herein, the term xe2x80x9cmixturexe2x80x9d refers to the combination of the paper-containing sample, the mediator, the oxidoreductase, oxygen, water, and optionally heat. The paper-containing sample may be dried prior to its inclusion in a mixture.
As used herein, the term xe2x80x9cincubatexe2x80x9d refers to the physical conditions to which the mixture is subjected which accomplishes a reduction of the fluorescence of the paper-containing sample. Incubation conditions include, but are not limited to, time, temperature, and pH.
In one embodiment, the pH of the mixture is from about 3.0 to about 6.5. In a preferred embodiment, the pH is from about 4.0 to about 5.0. In an even more preferred embodiment, the pH of the mixture is about 4.5.
According to the present invention, the mixture may be incubated from about 0.5 hours to about 20 hours. In a preferred embodiment, the mixture is incubated from about 1 hour to about 10 hours. In an even more preferred embodiment, the mixture is incubated for about 2 hours.
According to the present invention, the mixture may be incubated at an elevated temperature. In one embodiment of the present invention, the mixture is incubated at a temperature from about 35xc2x0 C. to about 60xc2x0 C. In a preferred embodiment, the mixture is incubated at about 45xc2x0 C.
In an embodiment of the present invention, incubation of a mixture wherein the paper-containing sample contains FWAs results in reduction of fluorescence of the paper-containing sample.
In still another embodiment of the present invention, incubation of a mixture results in the destruction of FWAs in a paper-containing sample.
After incubation, the mixture is thoroughly washed to remove the by-products of the reaction. Subsequent operations such as extraction and/or treating the pulp with bleaching chemicals may be performed to enhance the optical properties of the pulp. Extraction and chemical bleaching are both well known to those of skill in the art.
If desired, after the method of the invention has been practiced, a paper may be formed or reformed.